As the most popular process for obtaining multicolor images, silver salt photography has been widely put into practice from the viewpoint of high sensitivity, high image quality, excellent gradation reproducibility, etc. However, the silver salt photography has the defect that it requires complicated steps since it involves the steps of imagewise exposure, processing with a developer, and conversion of remaining silver halide to a water-soluble silver complex salt or to a silver salt stable to light.
In order to remove the above problems, JP-A-59-48764 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), etc. describe dry-process silver salt photographic light-sensitive materials; British Patent 249,530 and U.S. Pat. Nos. 2,020,775, 2,004,625, 2,217,544, 2,255,463 and 2,699,394, etc. describe dye diffusion transfer process photographic light-sensitive materials; and U.S. Pat. No. 2,844,574, etc. describe silver dye bleach process photographic light-sensitive materials.
On the other hand, as recording materials not using silver salts, there have been employed an electrophotographic system in which an apparatus is equipped with multicolor recording mechanism, a thermal transfer system, an ink-jet system, and the like. However, these systems have such deficiencies as that large-sized equipment is required, that recording reliability is insufficient, and that exchange of expendables is troublesome.
Since a thermal recording system does not involve the above-described problems, multicolor recording materials of that type of system would be quite easy to use. As a conventional process employing the thermal recording system, there has been a process wherein a plurality of color-forming units successively undergo color formation reaction as heat energy applied thereto is increased to give hues based on color mixing as described in JP-B-51-19989 (corresponding to U.S. Pat. No. 3,895,173) (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-52-11231, JP-A-54-88135, JP-A-55-133991, JP-A-55-133992, etc. As another process, there has been a process wherein a decolorizing mechanism of allowing a decolorising agent to act on a color-forming unit capable of forming color at a lower temperature simultaneously when a color-forming unit having a higher heat response temperature forms color is employed as described in, for example, JP-B-50-17868 (corresponding to U.S. Pat. No. 3,843,384), JP-B-51-5791, JP-B-57-14318 and JP-B-57-14319. However, these techniques have the defect that the number of hues capable of being formed is small, which is a fatal defect for a color hard copy.
As multicolor recording materials overcoming the above-described defects, recording materials wherein heat-sensitive coloration system, light-sensitive coloration system, etc. are combined are described in Japanese Patent Application Nos. 61-80787 (corresponding to U.S. Patent Ser. No. 07/174306), JP-A-63-172681, JP-A-63-45084 (corresponding to U.S. Patent Ser. No. 07/174306), JP-A-63-134282 (corresponding to U.S. Patent Ser. No. 07/125675), etc. These comprise a transparent support having provided on one side thereof at least one color-forming unit layer and on the other side at least one color-forming unit layer capable of forming a color of different hue from that of the above-described unit layer, said color-forming units containing a combination of a diazo compound and a coupler as a color-forming component and/or a combination of a leuco dye and a colordeveloper as a color-forming component. However, these recording materials unavoidably employ a transparent support for providing color-forming unit layers on both sides thereof because formation of a plurality of color-forming unit layers on the same side causes problems with respect to their shelf life, image density, etc. Therefore, there is a limitation that an opaque support cannot be used, and a disadvantage in producing them that an opaque layer must be additionally provided. In addition, upon formation of an image, thermal printing must be conducted from both sides of the recording material, and hence they have the defect of requiring a complicated recording apparatus.